[purplecinnamon]

So I understand that enthalpy is energy. But what energy specifically? Is enthalpy heat energy? Is it the energy that bonds atoms together and that's why it can either be released or absorbed? Or is that what Gibbs free energy is?

I'm trying to make sense of the equation:

ΔG=ΔH-TΔS

Conceptually, how is enthalpy different from Gibbs free energy?

Thanks!

[Corribus]

Maybe you will find this post useful:

http://www.chemicalforums.com/index.php?topic=81681.msg297434#msg297434

To expand on that briefly: the Gibbs energy is basically a way to anticipate whether a (chemical) process will occur spontaneously or not. The Gibbs energy is related to the concept of potential energy. If something has a high potential energy, it has a good chance of doing something useful - of moving to a state of lower potential energy. You are familiar with the concept of gravitational potential energy I'm sure (thinks fall down because of it). Well, the Gibbs energy is essentially a measure of chemical potential energy. Enthalpy is a portion of that, as chemical systems will tend to evolve toward a state that requires less energy to create. (Just like, it takes a lot of energy to lift something a hundred feet in the air, but comparatively less energy to lift it only twenty feet in the air; so something at a hundred feet in the air will tend to move to a space twenty feet in the air if given the opportunity). But chemical potential is not only limited to enthalpy, which is captured in the Gibbs equation. Entropy is also a consideration. This is a more abstract concept, but it's basically a measure of how many ways energy can be distributed. If you can distribute the energy over a larger area (for lack of better word), it's not as unfavorable as a situation where the energy is concentrated in once place. So if the energy can be distributed over many states, this can help compensate for having to expend that energy to create a new system. There's essentially a balancing act between how much energy it takes to change the properties of a system, and how concentrated that energy will be when you accomplish the change. This is why enthalpy and entropy have opposite signs in the Gibbs equation. The Gibbs energy incorporates both of these factors to help predict how a chemical system will evolve in time towards potential endpoints.

[Enthalpy]

Enthalpy is not linked simply with bonds, nor does it consist of heat.

Enthalpy is an energy that

• Does not fully reside in the considered body, as opposed to internal energy;
• But can be fully computed from the state of the considered body;
• Is very useful to compute heat fluxes and work. Tables give H rather than U.

Consider the extreme case of a liquid, taken non-compressible for simplicity. As it flows from a high to a low pressure, its internal energy doesn't change, but it can acquire speed or rotate a turbine. The change of P*V, here V*ΔP, lets us calculate this energy amount that U doesn't incorporate.

V*ΔP does not come from the liquid amount that passes through the turbine. The higher upstream pressure is provided by the liquid upstream, that has not passed through the turbine. Though, we can measure the P just before and after the turbine, at the liquid amount that passes through the turbine.

As a generalization, H adds PV to U because a part of the work or of the heat flux comes from outside the considered body hence is not internal energy. Then it applies to gases too, and in fact to any body.

Sidenote: thermodynamics applies to gases and liquids usually and historically, but is more general. Depending on the topic, U, H and the others can incorporate electric, magnetic, nuclear... energy to make useful predictions, about superconductors or nuclear equilibrium in multi-million-Kelvin plasmas.

[aga]

One major thing to remember about all thermodynamic calculations is that they make absolutely NO determination relating to Kinetics.

If the thermodynamic calculations say the reaction Will happen, those there atoms/molecules still have to meet each other before it even has a chance of happening.

Imagine some Fluorine in Space.
It certainly Will react with whatever it meets.
If it does not meet anything, it will not react, despite that fact that it certainly would.